Objective The technology of emulsified viscosity reduction is commonly employed to address the challenges of transporting heavy oil. However, the stability of heavy oil emulsions prepared with traditional emulsifiers is typically low, making them susceptible to demulsification due to external environmental factors during transportation. Previous studies have demonstrated the effectiveness of magnetic nanoparticles in enhancing the stability of oil-water emulsions, and the method of recycling through the application of a magnetic field. Nevertheless, traditional magnetic nanoparticles exhibit poor dispersibility and a high tendency to settle, which limits their ability to enhance emulsion stability and renders them vulnerable to external environmental influences.

Methods Hydrolyzed polyacrylamide (HPAM), characterized by its electronegativity and viscoelasticity, was introduced to modify Co₃O₄, a type of magnetic nanoparticle. Experiments were conducted to investigate the impact patterns and action mechanisms of various factors, including pH value, emulsifying temperature, and oil-water ratio, on the stability of the O/W emulsions of heavy oil prepared with Co₃O₄ before and after modification.

Results The stability of emulsions improved at higher pH values. Co₃O₄@HPAM exhibited a significantly greater impact on emulsion stability compared to Co₃O₄, with a water separation rate of only 2.13％ recorded after 4 hours for the emulsion at pH 10. The following factors were identified as contributing to the changes in emulsion stability. As the pH value increased, the absolute value of the zeta potential of the magnetic nanoparticles and the electrostatic repulsive force between oil droplets also increased. This led to a gradual rise in apparent viscosity and enhanced migration resistance of oil droplets in the emulsions, resulting in a steady decrease in the viscosity reduction rate, which, however, remained above 98％. Additionally, the oil-water interfacial tension decreased, facilitating the adsorption of magnetic nanoparticles onto the surface of oil droplets. Consequently, the oil droplets became smaller in size and were distributed more uniformly. As the emulsifying temperature increased, emulsion stability worsened. Specifically, at 25 ℃, the water separation rates of the emulsions prepared with Co₃O₄@HPAM and Co₃O₄ were respectively 25.00％ and 52.28％ after 4 hours. The apparent viscosity of the emulsions gradually decreased, and the viscosity reduction rates decreased, yet remained above 97％. The resulting decline in the strength of the oil-water interfacial film resulted in the coalescence of small oil droplets into larger ones. At an oil-water ratio of less than or equal to 7:3, an increase in the oil-water ratio corresponded to a higher apparent viscosity, a lower viscosity reduction rate, gradually enhanced stability, and an increasing number of oil droplets in the emulsions while becoming smaller in size. Notably, at an oil-water ratio of 8:2, the O/W emulsion was successfully prepared only using Co₃O₄@HPAM, yielding a water separation rate of 10.25％ and a viscosity reduction rate of 97.66％ after 4 hours.

Conclusion This study provides a comprehensive explanation of the promoting effect of magnetic nanoparticles on the stability of heavy oil emulsions under various experimental conditions. The findings can support the application of magnetic nanoparticles in heavy oil transportation technology that utilizes emulsified viscosity reduction.